The present invention relates to a transmission system comprising a transmitter having a channel encoder for encoding sequences of source symbols into sequences of channel symbols, the channel encoder being initialized into a state corresponding to the final state of the channel encoder after having encoded the sequence of source symbols, the transmitter being arranged for transmitting a channel signal representing the channel symbols to a receiver, the receiver comprising a channel decoder for deriving the sequences of source symbols from a channel signal representing the channel symbols by keeping track of a plurality of state sequences with a corresponding likelihood measure representing the likelihood of the state sequences.
Such transmission systems are used for transmitted sequences of source symbols in a reliable way to a receiver. Application of such transmission systems include mobile telephony systems in which frames comprising an encoded speech signal have to be transmitted from a base station to a mobile terminal and vice versa.
In order to achieve a reliable transmission, the sequences of source symbols are encoded into sequences of channels symbols by a channel encoder. The channel encoder can e.g. be a convolutional encoder which is initialized in a predetermined state. This predetermined state is often the zero state corresponding to the .nu. last symbols in the sequence of source symbols. A disadvantage of this is that the last symbols of the source symbols should be set to zero. This reduces the number of source symbols which can be transmitted in one sequence of channel symbols.
The reduction of the number of source symbols can be prevented by initializing the channel encoder to a state which is defined by the last source symbols in the sequence of source symbols. It is known to initialize the channel encoder in a state which is the same as the state of the channel encoder after the encoding of said last source symbols. Codes using the above mechanism are known as "tail biting" codes. Combinations of "zero tailing" codes and "tailbiting" codes are also conceivable.
It is however conceivable that the channel encoder is initialized in a state which is different from the state of the channel encoder after the encoding of said last source symbols. However in such a case there should be a one to one relationship between the state of the channel encoder after the encoding of said last source symbols, and the state to which the channel encoder is initialized before starting decoding.
A disadvantage of tailbiting codes is that the decoding complexity increases substantially, because the decoding should in principle be performed for each possible initial state. If the code has 32 possible states (.nu.=5), the decoding complexity is increased with a factor 32. It has been shown that it is possible to reduce the complexity by decoding the channel signal repeatedly in a circular fashion, but the complexity is still a few times the complexity of an decoder for a code initialized in a state known to the channel decoder.